Many production vehicles are equipped with traction control. Traction control prevents excessive drive wheel spin and therefore increases the stability and performance of the vehicle without compensation from the driver. Since wheel spin is due to a greater torque being applied to the wheels than the tractive limit of the tires for the given set of road conditions, a traction control event reduces the torque applied to the wheels.
Torque reduction is typically done by applying the brakes to the wheel or wheels that are spinning, as well as reducing the engine torque being applied. The engine torque reduction can be done several ways in a conventional vehicle: spark timing retard, fuel cutoff, and engine throttle. Of these alternatives, engine throttle is the most attractive when considering tailpipe emissions, fuel economy, NVH, and driver feel.
Unfortunately, engine transients, even with optimum AFR and spark timing, are a major contributor to tail pipe emissions due to physical and control time delays. Additionally, the engine must operate at less then an optimum condition during these traction control events. Also, in a conventional vehicle, the torque reduction due to each actuator must be calculated to determine the total torque reduction necessary from the engine. These calculations introduce errors and time to perform tasks may introduce significant wheel torque errors.
The disadvantages associated with this conventional torque control technique have made it apparent that a new technique for traction control for a hybrid electric vehicle is needed. Preferably, the new technique would allow the engine to continue operating at its optimum level during traction control events without negatively affecting tailpipe emissions, NVH or fuel economy.